Position-measuring devices which provide digital absolute positional values are being used more and more in the field of automation technology. Frequently, such position-measuring devices are also able to be programmed, i.e., they include memory units from which memory content is able to be read or to which memory content can be written. Memory content could be calibration data or configuration data, for example. Moreover, sensors for measuring additional parameters may also be included in the position-measuring device, for example, to measure the temperature. Digital data, especially the absolute positional values, are predominantly transmitted via serial data interfaces, since they are able to operate with relatively few data-transmission lines and yet achieve high data-transmission rates.
An important cost factor when connecting position-measuring devices to a control unit, e.g., a machine tool control unit, is the number of electric lines required for the operation, since this number significantly defines the price of the utilized high-quality data cables. For example, conventional synchronous-serial interfaces allowing a differential signal transmission (e.g., according to the RS-485 standard) require two lines in each case for the transmission of clock and data signals. Two further lines must be provided to supply the position-measuring device with energy. In other words, a total of six lines is required. An example in this regard is described, for instance, in European Patent No. 0 660 209.
In an effort to reduce the number of required lines while maintaining the differential signal transmission, which is especially advantageous because of its low interference susceptibility and the high achievable data transmission rates, German Published Patent Application No. 10 2008 027 902 describes dispensing with the transmission of a clock signal and implementing the data transmission via only one bidirectionally operated line pair. In other words, counting the lines for the energy supply, the lines are reduced to only four.